This invention relates to a process for the electrolytic recovery of a metal which is ionogenically contained in an electrolyte, wherein the electrolyte is supplied from a reservoir through at least one supply line to an electrolysis area including anodes and cathodes and at least one d.c. voltage source, and wherein used electrolyte is at least partly recirculated from the electrolysis area to the reservoir through at least one discharge line.
In electrolysis plants of this kind a so-called stray current usually flows through the supply line and the discharge line, which stray current leads to corrosion problems in the peripheral parts of the plant, e.g. in the reservoir, in the electrolyte conditioning and in a usually present electrolyte preheater. Would the supply line and/or the discharge line be grounded, metal deposits would occur in the line in the vicinity of the ground terminal. If one tried to solve these problems by current interruption, this would involve quite considerable costs.
It is the object underlying the invention to make the current flowing through the supply line and the discharge line ineffective in a simple and reliable way, so that even with relatively high electric voltages in the electrolysis area stray currents in the peripheral parts of the plant outside the electrolysis area are effectively avoided. In accordance with the invention this is achieved in the above-mentioned process in that between a first contact point in the electrolyte of the supply line and a second contact point in the electrolyte of the discharge line a bridge line containing electrolyte is provided, where the ohmic resistance R1 of the electrolyte in the bridge line between the first and the second contact point is not more than 10% of the ohmic resistance R2, which exists between the first and the second contact point in the electrolyte flowing through the reservoir, and that the amount of electrolyte flowing through the bridge line per unit time is not more than 5% of the amount of electrolyte flowing in the supply line in the vicinity of the first contact point.
Usually, the difference of the electric voltage in the electrolysis area between the supply line and the discharge line is at least 20 Volt, it may be lower but in particular also much higher. The problem of the stray currents increases with increasing voltage difference, and in the present case the bridge line provided is advantageous in particular when the voltage difference in the electrolysis area between the supply line and the discharge line is 100-800 Volt.
It should expediently be ensured that the ohmic resistance of the electrolyte flow in the supply line between the first contact point and the electrolysis area as well as between the second contact point and the electrolysis area is each at least 5 times and preferably at least 20 times the amount of R2. This can for instance be achieved in that the length of the line between the first or the second contact point and the electrolysis area is several meters and in particular 10 to 100 m.
It is ensured that the ohmic resistance of the electrolyte in the bridge line is as small as possible, so that the bridge line between the supply line and the discharge line wholly or nearly acts like an electric short circuit. At the same time it is important that the flow of electrolyte through the bridge line is small and possibly prevented at all. For this purpose, one or more flow obstacles are for instance incorporated in the bridge line, but at the same time a continuous electrolytic wetting exists. For the flow obstacle there may for instance be used a bed of isolating granules, e.g. ceramic or plastic beads, nets, a knitted fabric, a sponge-like plug, a diaphragm or an ion exchanger membrane, in particular an anion exchanger membrane. Furthermore, a control valve may be provided in the bridge line, by means of which control valve the desired small electrolyte flow rate can be adjusted.
The electrolysis may serve the recovery of copper, nickel, zinc or cobalt, where the electrolyte solutions known per se are employed. Details of the configuration of an electrolysis used for metal recovery are known and described for instance in Ullmann""s Encyclopedia of Industrial Chemistry, 5th edition, vol. A9, pp. 197-217.